1. Field of the Invention
The invention pertains generally to contrast enhancement filters for viewing color displays, and more particularly to absorption filters providing improved viewability under conditions of high ambient light levels.
2. Description of the Prior Art
When utilizing many displays such as cathode ray tube displays in high brightness ambient conditions, the information on the display becomes difficult to read due to reflections from the viewing screen. In such high ambients, reflected light from the viewing surface can exceed the internal light generated by the display itself, thereby resulting in a sufficient loss of contrast to wash out the presented information.
One known prior art type of filter uses neutral density filtration. This has been achieved by providing colloidal suspensions such as silver in the glass composition of the faceplate or the implosion panel, or a colloidal graphite suspension within an adhesive material used to bond the implosion panel to the faceplate, as in U.S. Pat. No. 3,879,627. This type of filter transmits a fraction of the light passing through it, which fraction is independent of the color emissions (i.e., wavelength). Since the light from the display passes through the filter once, while ambient light reflected from the display must pass through the filter twice, the ratio of display brightness to ambient background brightness is thereby enhanced.
When exposed to high brightness ambient lighting conditions, such as bright sunlight, the attenuation of transmitted light by a neutral density filter that is required to provide a satisfactory contrast ratio is so high that the resulting display brightness is too low to be acceptable. Another approach is selective filtration, obtained, for example, by the use of colored glass for either the tube faceplate or the implosion panel, or by adding colorants to the bonding material between the faceplate and the implosion panel. Such an approach is useful for a monochrome display, where the absorption characteristics of the filter may be formulated to provide a narrow band of transmission at the primary display wavelength. As with the neutral density filter, contrast is enhanced by the greater losses suffered by the ambient light in its reflected passage through the filter, as against a single passage through the filter of internally produced display light of monochromatic wavelength. However, at other wavelengths, the narrow band filter absorbs substantially all the ambient light. Therefore the contrast ratio, defined herein as the ratio of display brightness plus background brightness to background ambient brightness, is much greater with a narrow band filter than with a neutral density filter. However, this approach requires a different colorant for each type of phosphor screen.
Optical filters utilized by the prior art for color displays include absorption filters and thin film filters. An absorption filter is herein defined as one adapted to pass a narrow spectrum of light and substantially absorb all others. Such filters are relatively independent of the viewing angle. Color displays, such as are provided by color cathode ray tubes, typically emit three primary color wavelengths, such as red, green, and blue, which may be combined to derive many colors. The ideal filter for a color CRT is comprised of three individually operative pass bands, each transmitting a narrow band of red, green or blue wavelengths emitted by the display. Materials are known which permit reasonably selective transmission of each of these wavelengths individually. However, when such materials are either mixed in a single layer, or when individual filters for each wavelength are cascaded, such filters will mutually absorb wavelengths transmitted by the others. In the prior art, selection of colorants and materials to provide selective transmission of multiple colors has been hampered by the co-existent requirements that each constituent element must transmit a predetermined narrow band of primary color wavelengths, yet not unduly absorb another wavelength band. Satisfactory materials with the precise desired transmission and absorption characteristics have not been found. The best of such prior art filters, as commonly used in aircraft for color CRT displays, has far from the ideal absorption spectrum.
A further prior art approach that does not rely on absorbing materials to produce the desired transmission characteristics uses complex multilayer optical thin films or holograms. The effectiveness of these types of filters is reduced by a strong angular dependence inherent in the filter. The transmittance and reflectance properties are determined by interference phenomena which depend on optical path length, which in turn depends on the viewing angle. Therefore, these filters are appropriate only in systems whose viewing angle is tightly constrained.
Yet another approach to improve contrast is simply to increase the brightness level of the displayed information. However, thermal and longevity limitations preclude increasing brightness in conventional raster color displays to levels that are readable in daylight. A substantial increase in brightness can be obtained by operation in a vector (or stroke) writing mode. Vector writing has the disadvantage of being less efficient in terms of power consumption and the type of information which can be displayed.